Coated electro-luminescent devices, for example electro-luminescent devices employing coated organic light emitting diode materials as discussed by U.S. Pat. No. 4,769,292, issued Sep. 6, 1988 to Tang et al., and U.S. Pat. No. 5,061,569, issued Oct. 29, 1991 to VanSlyke et al, have been discussed for creating large, diffuse lights for providing general illumination. Cok in U.S. Pat. No. 6,776,496, entitled “Area illumination lighting apparatus having OLED planar light source”, discusses one example of such a lighting device. Since these devices are among the only luminance sources known that can be manufactured by coating large substrates with materials that may be electrically energized to produce large, uniform lamps, they hold significant promise for producing large illumination sources that can provide diffuse illumination from a large area source, providing relatively even illumination for large rooms. This ability to create large diffuse illumination sources is considered one of the primary advantages offered by this technology, although they have other advantages, such as high efficiency.
Such lamps can provide diffuse, relatively even illumination over large environments. While it is often the case that users require a relatively low level of illumination within their general environment to allow them to navigate a space and to create a generally desirable environment, users often require a higher level of illumination to perform detailed tasks, such as reading. Therefore, multiple lighting sources are often used in many environments, including light sources that provide large area environmental lighting and additional light sources that provide higher levels of task illumination. It is true that the luminance of the large area illumination sources could be increased to provide a high level of luminance across the entire environment to facilitate a task, such as reading, however this would result in unnecessarily high illumination levels across a very large proportion of the environment and will likely result in significantly large amounts of wasted energy.
It is known in the field of electronic displays to provide directed light output. For example, Lee in US Publication 2007/0091037, entitled, “Energy Efficient Compact Display for Mobile Device” discusses an electronic display coupled with an array of micro-lenses, wherein a relatively large array of addressable light-emitting elements are located beneath each micro-lens. Within this display system, the light is directed to one or more locations in space by selecting one or more of the light-emitting elements within the array of light-emitting elements. As such, the light provided by the display may be directed to a small proportion of the 180-degree hemisphere in front of the display. One of the primary advantages of such a design when employed with an electro-luminescent display is that only the active light-emitting elements consume power and therefore, such an arrangement can be used to direct light to the user. Therefore, the arrangement provided by Lee, who also discusses tracking the user's movements, allows the user to perceive that the display emits light in all directions even though only enough light is generated to provide this level of illumination within a small proportion of the 180-degree sphere in front of the display. Unfortunately, such a design does not simultaneously provide both a uniform illumination across a large environment and a task light. Further, the ability to address a very large number of light-emitting elements with different signals to create such a display device will typically require complex drive circuitry, typically including active matrix drive backplanes. Unfortunately, such backplanes are not applicable to general-purpose lighting devices due to the high cost of such backplanes with respect to the selling price of such general-purpose lighting devices.
It is also known to employ micro-lenses to improve light extraction from thin film electro-luminescent display devices, such as OLEDs. For instance Moller in U.S. Pat. No. 6,984,934 entitled “Micro-lens array for display intensity enhancement” and Lu in U.S. Pat. No. 7,053,547 entitled “Increased emission efficiency in organic light-emitting devices on high-index substrates” discuss placing lens arrays over a device having a small-molecule organic emissive layer to increase the light extraction efficiency of the device. However, these lens arrays are not used to direct the light but only to diffract the light to more efficiently couple light out of a high index material into air. Therefore, it is important that these lenses be placed in direct contact with high index materials that are in direct contact with the light-emissive layer, which precludes the use of these lenses to direct the light to defined locations in space.
It should be noted that since the lens arrays for providing light diffraction to aid light extraction from a high index electro-luminescent device does not direct the light but simply diffract the light, the shapes of the lenses are not particularly important. In fact, this function may be performed to some degree by creating any diffraction pattern, which redirects the light. Therefore relatively simple manufacture of such lens arrays have been discussed by others techniques, which do not precisely control the tolerances on the lens shape. One such method has been discussed by Sakai in U.S. Pat. No. 6,952,311, entitled “Microlens device and method of manufacturing the same, electro-optic device and electronic apparatus”. Note that this method requires only the ejection of droplets onto a substrate, followed by a curing step and that any changes in surface properties between neighboring drop sites will significantly affect the drop shape. Therefore, the methods used to create such microlens arrays will not be applicable to the formation of microlens arrays that can be used to direct light from the source to a defined location in space as would be required to provide the function as described in US Publication 2007/0091037.
There is, therefore, a need to provide a general-purpose light source that can provide both large area illumination and simultaneously provide task lighting into a more localized area. Such a device must be provided with low cost to make it applicable to the general-purpose lighting market.